Software defined radios (SDR) strive to digitize radio frequency (RF) signals and perform spectrum sensing in the digital domain. SDRs perform this spectrum sensing to determine where available or unused spectrum and unavailable or used spectrum resides in order to provide spectrum for use in communicating an RF signal. However, attempting to perform spectrum sensing for wideband inputs often translates to impractical or potentially infeasible analog-to-digital converters (ADC) specifications.
As a result of these impractical ADC specifications, a number of techniques, including time interleaving and N-path filter-banks have been proposed to address the wideband channelization problem. However, while time interleaving ADCs may reduce ADC speed, the input dynamic range (exponentially related to ADC power) may remain large. Filter-banks may reduce both the speed and dynamic range (by removing out-of-band signals) of ADCs. However, filter-banks are commonly based on phased-locked loops (PLLs), which are a type of control system that generates an output signal whose phase is related to the phase of an input “reference” signal, mixers and low-pass filters, each of which may consume considerable amounts of power. Additionally, signal reconstruction from the digitized filter-bank outputs may be challenging.
In addition, spatial diversity to achieve signal directivity and filtering can be achieved using phased arrays. Such techniques can be applied to a variety of applications to improve communication robustness and performance. Applications may include use by the military to send and receive information from fixed directions and to filter out enemy blockers. Other applications may include usage in automotive and vehicular radios, satellite communications, imaging applications, home audio/visual applications, and the like.
In addition, it may also be possible to use additional diversity to improve communication robustness and performance. For example, in addition to using spatial diversity for precise targeting, radar systems may also use frequency diversity to reduce target fluctuations. Furthermore, diversity in space and time may also be used to send and receive codes occupying the same frequency bandwidth. With each degree of diversity an additional degree of freedom may be gained to manipulate and immunize signals of interest.